Electron emitting cathode and method of preparation



Patented May 3, 1938 UNITED STATES ELECTRON EMITTING CATHODE AND METHODOF PREPARATION Charles H. Prescott, Jr., East Orange, N. 3., assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York No Drawing.

Application April 13, 1935,

Serial No. 16,260

11 Claims.

This invention relates to electron emitters and more particularly tocomposite coatings for said emitters and methods of preparing andactivating the emissive coating matrix.

6 The oxide coated type cathode has been commercially practical for anumber of years. Technical advances have materially increased its lifeand efficiency in operation and portrayed the complex character of theconstituents of the matrix structure of the coating. However, such acathode, usually called a dull emitter, is limited in use by the currentdissipated in the cathode and the power rating of the device in which itis used. In devices of high power consumption, it is more practical toemploy a tungsten filament or a thorlated filament. The latter typefilament, usually referred to as a bright emitter, is generally preparedby an .ex trusion process in which the constituents are 20 fabricated inthe core of the filament and the active agent, thorium, diffused to thesurface of the core. Such an emitter does not embody the complex matrixstructure of the dull emitter.

An object of the invention is to control and 25 reproduce the electronactivity in emitters of the thorlated type.

Another object of the invention is to reproduce the structuralcharacteristics of the dull emitter in a bright emitter and attainincreased 30 life and efliciency in operation.

A further object of the invention is to activate composite type emittersby direct chemical methods.

In accordance with this invention a metallic 35 core or base capable ofserving as a current conducting body is provided with a coating matrixincluding thorium as the active emitting component, a stable highlyrefractory metal, to serve as a repository component and a mass of inertseparating material, preferably a compound of the active metal, such asthorium dioxide.

Aspecific example of an emitter according to this invention consists ofa core or base of tung- 4 sten having a composite matrix coating of alarge mass of thorium dioxide, in which finely divided particles ofmolybdenum, iridium, tungsten or similar metals are dispersed with anadsorbed layer of thorium on the surface of the coating and 50 a supplyof thorium associated or alloyed with the repository particles of thestable metal. This composite emitter exhibits the uniformcharacteristics of dull emitters both for operating lifeand;efficiency,'enables a wide range of control 55 over theproportioning of the constituents of the matrix and provides an emitterwhich is easily reproducible for mass production.

The composite emitter may be activated in a hydrocarbon gas at a lowpressure or, by including carbon or a carbonaceous compound in the 5matrix coating, to reduce some of the thorium dioxide to thorium metalto serve as the active component of the coating.

A feature of the invention is an activation process involving-thechemical reduction of the highly refractory compound of active metal bya reducing agent incorporated in the coating matrix. The only necessarytreatment is to heat the emitter in vacuum.- This promotes a rapid andeconomical manufacturing process which is, furthermore, substantiallycontrolled by the coating composition so that the product is of auniform quality.

Another advantage is that this matrix may be applied and activatedindependently of the chemical constituents of the emitter base, itsmechanical configuration or the relative configuration of the emitterand other electrodes or parts of the device.

Further features and advantages will appear in the following detaileddescription.

The composite matrix coating of this invention may be prepared byforming a mixture of a large mass of thorium dioxide with a small amountof finely divided particles of a highly refractory stable metal, such asmolybdenum, tungsten, iridium, osmium, ruthenium, tantalum, hafnium, orrhenium, together with a reducing agent, such as carbon or acarbonaceous compound, preferably powdered graphite, cane sugar orlampblack. The mixture of the thorium compound, the stable metalparticles, and the reducing agent may be commingled in a binder materialof nitrocellulose and amyl acetateto form a viscous fluid suitable forcoating. Instead of using the finely divided metallic particles of thestable metal, it may be more desirable to employ a compound or salt ofthe stable metal. For instance, molybdenum oxide is a satisfactorycompound for the coating mixture. In another form of the invention whenthe activating or reducing agent used is cane sugar it may be moredesirable to use water as the coating vehicle instead of the amylacetate.

After the coating mixture is prepared it may be applied to a suitablebase metal, such as tungsten, which is capable of carrying current ofhigh amplitude without deleterious efiects on the coating matrix appliedthereto. After a coating of suitablethickness is secured on the base thecoating sel with the emitter, for instance, a control electrode or'grid,and an anode or plate may bemounted in suitable spaced relation withrespect to the emitter to form a complete discharge structure. However,since this invention is primarily concerned with the development of thecoating matrix and the activation of the coating material to produce anelectron emitting substance 'in and on the matrix, the followingdescription will set forth the various steps for attainingv this resultand performing the direct chemical reduction for producing the electronemitter of this invention.

The preliminary procedure after the assembly of the cathode and otherelectrodes in the enclosing vessel, consists in sealing the vessel to anevacuating station, baking the glass vessel to rcmove water vapor andfollowing this step with a, thorough pumping of the vessel to remove allgases and other deleterious matter which might impair the succeedingsteps of the process. when a low pressure is obtained in the vessel,generally of the order of 2x 10- mm. of mercury, the initial processingof the cathode may be begun.

The technique of the process may be varied according to the initialmaterial incorporated in the coating matrix. For instance, if thestarting material of the matrix consists of thorium dioxide, molybdenumoxide and powdered graphite, it may be desirable to reduce themolybdenum oxide to'metallic molybdenum by heating the emitter orcathode in hydrogen to a temperature above 1,000 C. to effect thereduction without loss of carbon and leave as. a residue the finelydivided particles of metallic molybdenum. The hydrogen is removed fromthe vessel by the pumps. The molybdenum metallic particles, after thereduction treatment, are quite stable since the melting point of thismetal is sumciently above the activation temperature encountered infollowing this invention and may be relied upon to maintain its metallicform during the succeeding steps of the process. It may be desirable toemploy other compounds of molybdenum, such as ammonium molybdate. Theconversion step maybe eliminated entirely by incorporating finelydivided particles of molybdenum in the coating suspension. In a similarmanner, compounds of other stable metals, such as the oxides oftungsten, tantalum, hafnium or rhenium or ammonium salts, such as,tungstate', tantalate or chloroirldate, may be employed in the coatingmixture. It is not essential that these compounds be reduced to themetallic form prior to continuing the activation processof thisinvention as adequate carbon may be employed to form by reduction boththe stableand the active metals. But any compounds of the platinummetals will decompose spontaneously at relatively low temperatures. Theprime requisite of the stable metal is to serve as a depository surfacefor the thermionically active metal, when developed, and this metal hasa high melting point, which is sufliciently above the activationtemperature, that it a,11s,sas

is not affected b subsequent mnnent of the matrix and is sumcientlystable to function of a repository material.

The cathode is then heated to an activating temperature of 1700 to 2200'C. to cause the powdered graphite in the matrix coating to react withsome of the large mass of thorium dioxide in the matrix and reduce aportion of the thorium dioxide according to the following equation:ThOz+2C= Th+2CQ If the molybdenum oxide is similarly reduced theequation will be MoOa+3C -=M0+3CO. The carbonmonoxide is evacuated bythe pumping apparatus and the filament is'heated to difluse the activethorium throughout the matrix, that remaining near or at the surfaceserving as the primary source of electron emission for the cathode. Someof the active thorium'within the matrix associates or alloys with thestable metal particles in the matrix or is adsorbed upon thesurfacethereof and forms a reservoir supply to replace the evaporated surfacemetal during the operation of the emitter.

In place of powdered graphite other reducing agents may be. substituted,such as lampblack or cane sugar. When cane sugar is the reducing agentand also incidentally the binder material perform the -of the matrix, itis desirable to carbonize the sugar at a lower temperature prior to theactivation process in order to avoid a too rapid evolution of gases.

After the activating treatment has been completed and a high vacuumsecured in the device by pumping out the residual gases generated duringthe activatingperiod the device may be finally sealed oil! the pumpingstation. However, in order to stabilize the activity it may be desirableto reheat the cathode to a temperature of about 1700 C. for a period ofone-half hour. This treatment facilitates the disperson of the finelydivided stable particle metal and the associated active thorium metaladhering to the particles of the stable metal. It will be apparent thatthe preparation of the cathode in accordance with this inventionproduces a thoriated type emitter for use in high power devices havingcharacteristics simulating those of the dull emitter. Furthermore, theactivation of such an emitter may be effected by direct chemical methodsin accordance with this invention to produce an emitter having highefllciency and long life andv capable of filling a gap in the range ofuses heretofore unattainable with the well-known oxide type emitter andthe thoriated type emitter. Furthermore, the proportions of the coatingmatrix may be more easily controlled than is possible with the alloytype in which the thorium is diffused from the core.

The complex matrix type emitter of this invention may also be activatedby a method disclosed in U. 8. Patent 2,019,504, to C. H. Prescott, Jr.,issued November 5, 1935, in which a hydrocarbon gas'of the paraffinseries, namely, methane, is introduced into the vessel under reducedpressure to activate the emitter substance and form a reservoir supplyin the coating matrix. In the later method, it is to be understood thatthe reducing agent of this invention may be omitted from the coatingsolution and the gas carbonizing substituted in accordance with theabove application.

2. A coating composition comprising thorium dioxide, powdered graphite,and a molybdenum compound.

3. A coating composition comprising thorium dioxide, cane sugar, andmolybdenum oxide.

4. An electron emitter comprising a refractory metallic base, acomposite matrix coating thereon including a mass of thorium dioxide,finely divided particles of a stable refractory metal dispersedthroughout said mass, and a refractory electronically active metaladsorbed on the surface of said mass and associated with said stablemetal particles.

5. An electron emitter comprising a core having a composite coatingcomposed of thorium, molybdenum, and thorium dioxide.

6. A thoriated emitter comprising a core of tungsten, a composite matrixcoating thereon including a mass of thorium dioxide, finely dividedparticles of molybdenum, and metallic thorium adsorbed on the surface ofsaid mass and alloyed with said molybdenum particles in said matrix.

7. A method of activating an electron emitter coating comprising acompound of molybdenum, a reducing agent, andthorium dioxide, whichcomprises heating said coating in vacuum, causing a chemical reaction toform metallic molybdenum and metallic thorium, and associating saidthorium with said molybdenum.

'8. A method of activating an electron emitter coating comprising acompound of molybdenum,

a reducing agent, and thorium dioxide, which comprises reducing saidmolybdenum compound to metallic molybdenum, heating said coating invacuum, causing a reaction between said reducing agent and said thoriumdioxide to produce metallic thorium, and associating said thorium withsaid metallic molybdenum.

9. A method of activating an electron emitter coating comprisingmolybdenum and thorium compounds which comprises heating said emitter todecompose the compounds to oxides, reducing the molybdenum oxide tometallic molybdenum, heating said emitter in a hydrocarbon atmosphere toreduce a portion of thorium oxide to metallic thorium, and alloying thethorium with the molybdenum in vacuum by heating. I

10. A method of activating an electron emitter coating comprisingmolybdenum oxide dispersed throughout a matrix of thorium dioxideincluding powdered graphite which comprises reducing said oxide tometallic molybdenum, heating said emitter in vacuum to chemicallycombine said graphite with a portion of said thorium dioxide to formfree metallic thorium, and adsorbing said thorium upon said metallicmolybdenum particles.

11. A method of activating an electron emitter coating comprisingmolybdenum oxide dispersed throughout a matrix of thorium dioxideincluding powdered graphite which comprises reducing said molybdenumoxide to metallic molybdenum, heating said emitter in vacuum, forming areaction between said graphite and a portion of said thorium dioxide toform free metallic thorium diffusing some of said thorium to the surfaceof said matrix and associating the remainder of thorium with the finelydispersed particles of molybdenum in said matrix.

CHARLES H. PRESCOTT, JR.

